The Blood gang uses this as a kind of call to alert fellow Bloods that either they need them for something or just to establish that they see eachother like " Whaddup Blood." You have to roll your toungue when you pronounce this.
The rival gang Crips also sometimes use the word "Clatt" as a form of disrespect to the bloods and to also alert their fellow crips too.

I saw my b^rother yesterday reppin that red, so I yelled "blatt" to him and he threw up them signs ya digg.

This is straight from Russian: "Blatt" is slang for a hooker. It is also a very insulting curse word, regardless of it's meaning, and a word that is used to express frustration or anger (much like "Fuck" in English). Since it's use is so common by some who speak Russian, it became very popular and common in Hebrew slang as well, for the same reasons described under Gandon.

"Blatt! I can't open this box..."

"Blatt! He won't answer the phone!"

"And then Blatt I went to the store Blatt and told the clerk Blatt give me Blatt the goddamn chewing gum Blatt!!"
(extreme example)

To be extraordinarily and unnecessarily uncomfortable giving up a possession even when you don't even need it. Also, finding a way to get involved in something that you're not part of through some deceptive, sneeaky or slimy methods.

Your friend only has one soda left and is very thirsty, and you ask him for it: "Hey give me your last soda." "It's my last one and I'm really thirsty; Stop being so Blatt." Also, you're about to throw away a notebook because the semester is over, you friend asks you for it to use, and you say no... i'm throwing it out anyway.